Why not a 50-135mm zoom lens?

Arthur Dent

Link Posted 24/10/2005 - 22:22
Minolta, Nikon and Canon have all at one time offered a 50-135mm zoom lens. I wonder why Pentax never did?

It would be a perfect zoom for the digital SLR's with the APS-C sensor. It would cover basically the same angles as the 70-210 did with the full frame cameras.

How about it, Pentax, it could be a big seller!

Kim C

Link Posted 24/10/2005 - 22:27
Maybe not a 50-135 but they have done a 45-125. I know, I have one! http://www.bdimitrov.de/kmp/lenses/zooms/medium/K45-125f4.html
They also do a 50-200 which covers this range specifically for the digital sensor http://www.bdimitrov.de/kmp/lenses/zooms/medium/DA50-200f4-5.6.html

George Lazarette

Link Posted 25/10/2005 - 00:29
There's also the 35-135mm zoom, which on film is a very good "tourist" lens if you don't want to weigh yourself down. The 24-90mm has an equivalent range on digital, and I find it very useful.


Arthur Dent

Link Posted 25/10/2005 - 02:53
Thanks for the info, I'll start searching right away!

I'm sure they must turn up on ebay once in a while.

It's a fixed apeture zoom, which is the only kind I'll use (flash meter and studio lights, and all that).


Link Posted 25/10/2005 - 08:38
G'day All.
I use Pentax SMC F 35-135 on the *ist Ds with very good results.

Slightly soft wide open but sharp @ 5.6 and above at all zoom ranges.

I have another one that Lives on the PZ-1p full time.
Again very good alround lens.
Regards, Trevor
be it a Glock or a Pentax, Any day shooting is a good day.

George Lazarette

Link Posted 25/10/2005 - 11:48

I find that in the studio I'm usually using f8 or f11, so the fact that a zoom is not fixed-aperture doesn't matter. It only matters when you are using it wide-open.

If you ARE shooting wide-open in order to minimise DOF, then surely a fast prime would be a better choice.


Arthur Dent

Link Posted 25/10/2005 - 14:15
George, are you saying that if the variable-apeture zoom is set to a smaller apeture than the maximum, it will remain constant while zooming? I've seen lenses where not only the maximum apeture changes during zooming but so does the minimum apeture. That leads me to beleive that the apeture blades do not move at all while zooming, thus making all apetures variable.

More information would be appreciated.

I still want that 45-125, though!

Kim C

Link Posted 25/10/2005 - 15:05
I think it will depend on what lens you are using. On the "traditional" fixed zoom such as the K's and A's the aperture blades move as you zoom at all apertures to maintain the constant aperture. They had to do this because the older meter systems were simple. What happens on the J lens and DA, I am not sure. With a modern body, the zoom length is transmitted to the body and indeed used by the modern autozoom flashes to move the head. As the body stops the lens down, it should be able to compensate at apertures less than the max. I can't think od an easy way to check this as you can not set a manual aperture and watch the blades as you zoom. In any case is you are using a zoom in a studio setup, I would think that you are usinga smaal part of the zoom range to get tight cropping. You should be well aware of the rough area of the range you are in and could make adjustments. The change over a small range would be well within the exposure latitude. As the 45-125 is a K lens, this might be easier on a Digital.


George Lazarette

Link Posted 25/10/2005 - 21:34

I am not an expert, but I believe any decent zoom lens will increase or reduce the size of the actual aperture as you zoom so as to maintain a constant aperture value.

In any event, whatever the focal length, if the lens says f8, it must be f8; it can't be f16. If that were so, the markings on the aperture ring would be useless, and photography with a zoom lens on a manual exposure camera would be impossible.

The only problem arises when you are shooting wide, with the lens on max aperture. Zoom in, and the lens WILL alter the aperture to whatever is the maximum posible at that focal length.



Link Posted 25/10/2005 - 22:46
There are two types of aperture correction for zooms - mechanical and optical. The pentax zooms that I have with constant f numbers are all of the mechanically compensating type - that is, as you zoom out to wide angle the aperture closes down slightly. Optical correction is also possible, but I don't know of any examples of this.

If you do not change the configuration as you zoom, you end up with a changing aperture value, and there are lots of zoom lenses around that have variable f numbers.

Think about it this way - if a 50mm lens has a front element diameter of 25mm then it will be f2 (50 divided by 25). If the lens could become a zoom and was zoomed to 100mm it would be f4 (100 divided by 25).
Best regards, John

George Lazarette

Link Posted 26/10/2005 - 00:01

Sorry, I didn't find that very helpful. What are you trying to say?

The question is this: when a zoom is set to f8, is it f8, or is it something else? If it IS f8, regardless of focal length, then some compensation is occurring. If it is NOT f8, then:

1 how doyou know what the aperture is?
2 why do they bother to inscribe apertures on the barrel if they're not correct?
3 how do you know what shutter speed to set on a manual camera if you don't know the aperture?

It seems pretty obvious to me that ALL proper zooms contain a compensating mechanism in them to ensure that the aperture is what the lens says it is, whatever the focal length (except within the stated max aperture range).

That being the case, the "variable aperture" bit only refers to the range between the largest and smallest max aperture stated in the lens description (eg: 28-80mm f4.5 - f5.6), and only at longer focal lengths.

Provided you use an aperture no larger than the smallest max aperture (f5.6 in my example) the aperture will not change.

Is that correct or not? Logic says yes.

Keywords: Charming, polite, and generally agreeable.


Link Posted 26/10/2005 - 00:21
Hi George

Tricky, but here goes:

1. In a variable-aperture zoom design, there is no compensation as you zoom. This is cheaper to manufacture and may not matter with auto exposure cameras, but it will matter with manual exposure. The aperture changes continuously, but only the extremes are quoted. Some zooms have small lines etched to mark out the degree of discepancy from the marked value.

2. We have to have something marked on the barrel otherwise there would be no clue whatsoever as to what the aperture was. The widest aperture is usually inscibed. If we were being really pedantic, a very complex zoom will have some light loss so a physical aperture of say f8 will have a transmission aperture slightly less, usually referred to as a T stop. T stops are not used in still cameras these days, but were once upon a time.

3. It's close enough, but you're right that there is a discrepancy and the usual advice is to meter after zooming. If you zoom again, the metering will change.

We get into some interesting territory in these discussions. Of course part of the problem is always that the more deeply one goes into something, the less true many practical truths become.....

Hope that is more helpful.
Best regards, John

Kim C

Link Posted 26/10/2005 - 00:27
Hi George,
I believe John is correct. The aperture scale on a variable aperture zoom is only correct at it's widest setting. Unless there is a built in system such as in the 24-50/F4 M or A lens to alter the aperture blades, the effective aperture will change. Take a variable aperture zoom such as you mention, point it at an evenly lit wall and select aperture priority and F8. As you zoom you will find that the camera will change the shutter speed. If as you suggest, it remains at F8 then the speed should stay the same as the exposure hasn't changed.

Mostof the early zooms and the more expensive present day ones such as the 28-70/2.8 have fixed aperture to prevent this. As Auto exposure cameras became more normal, the camera manufacturers kept the cost down by introducing variable aperture lens as the automatic exposure would compensate. For Pentax, this really started in the A range. The more expensive ones were fixed aperture but the budget ones were not.

I have just tried this on my MZ-s with the 28-90/3.5-5.6. At 28mm, the camera said the right exposure was 1.5secs and the aperture was F8. As I zoomed out it changed in steps until at 90mm it said 5 secs at F13. This is the same 1 1/2 stops difference in aperture that it is at wide angle. At least the camera tells me what the effective aperture is even if the lens does not.



Kim C

Link Posted 26/10/2005 - 00:27
Oops, I must have been typing as John was


George Lazarette

Link Posted 26/10/2005 - 00:52
Well, I've just done an experiment with the following lenses:

1 M 100mm 2.8
2 FA 100mm 2.8 macro
3 M 75-150mm 4 zoom
4 F 35-150mm 3.5-4.5 zoom
5 F 70-210mm 4-5.6 zoom
6 VS1 70-210mm 2.8-4 zoom

Camera to manual, zooms all set to 100mm, apertures 5.6, then 8 and then 16. All lenses tested in order at the same aperture, then the next, etc. Speeds fixed at start of each test to give a correct exposure for the first lens to be tested.

Result: Very consistent exposures for all the Pentax lenses; no discernible differences between primes and zooms at any of the tested apertures. No sign of any aperture problems with the zooms.

Surprise. The Vivitar (a proper Series 1 by Komine, said to be the best) was a good one stop down at each aperture, compared to the Pentaxes.

I should of course have tested the same zooms against primes of different focal lengths, but I didn't have time.

What this tells me is that, on the evidence of this limited test, so-called variable aperture zooms adhere closely to their stated apertures.

I think the point may be that these lenses are not what a textbook might call a variable aperture zoom. They are zooms which have a different max aperture at different focal lengths, but have a consistent aperture otherwise.

Keywords: Charming, polite, and generally agreeable.
Add a Comment
You must be registered or logged-in to comment.